Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-Carrier Frequency Division Multiple Access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE (Release 8).
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Medium Access Control (MAC) and MAC Control Elements
The MAC layer is the lowest sub-layer in the Layer 2 architecture of the LTE radio protocol stack (see 3GPP TS 36.321, “Medium Access Control (MAC) protocol specification”, version 8.7.0, in particular sections 4.2, 4.3, 5.4.3 and 6, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The connection to the physical layer below is through transport channels, and the connection to the RLC layer above is through logical channels. The MAC layer performs multiplexing and demultiplexing between logical channels and transport channels. The MAC layer in the transmitting side (in the following examples the user equipment) constructs MAC PDUs, also referred to as transport blocks, from MAC SDUs received through logical channels, and the MAC layer in the receiving side recovers MAC SDUs from MAC PDUs received through transport channels.
In the multiplexing and demultiplexing entity, data from several logical channels can be (de)multiplexed into/from one transport channel. The multiplexing entity generates MAC PDUs from MAC SDUs when radio resources are available for a new transmission. This process includes prioritizing the data from the logical channels to decide how much data and from which logical channel(s) should be included in each MAC PDU. Please note that the process of generating MAC PDUs in the user equipment is also referred to a logical channel prioritization (LCP) in the 3GPP terminology.
The demultiplexing entity reassembles the MAC SDUs from MAC PDUs and distributes them to the appropriate RLC entities. In addition, for peer-to-peer communication between the MAC layers, control messages called ‘MAC Control Elements’ can be included in the MAC PDU.
A MAC PDU primarily consists of the MAC header and the MAC payload (see 3GPP TS 36.321, section 6). The MAC header is further composed of MAC sub-headers, while the MAC payload is composed of MAC Control Elements, MAC SDUs and padding. Each MAC sub-header consists of a Logical Channel ID (LCID) and a Length (L) field. The LCID indicates whether the corresponding part of the MAC payload is a MAC Control Element, and if not, to which logical channel the related MAC SDU belongs. The L field indicates the size of the related MAC SDU or MAC Control Element. As already mentioned above, MAC Control Elements are used for MAC-level peer-to-peer signaling, including delivery of BSR information and reports of the UE's available power in the uplink, and in the downlink DRX commands and timing advance commands. For each type of MAC Control Element, one special LCID is allocated. An example for a MAC PDU is shown in FIG. 21.
Power Control
Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment. In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells. The role of the Power Control (PC) becomes decisive to provide the required SINR (Signal-to-Interference plus Noise Ratio) while controlling at the same time the interference caused to neighboring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, the 3GPP has adopted the use of Fractional Power Control (FPC) for LTE Rel. 8/9. This new functionality makes users with a higher path-loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
The power control scheme provided in LTE Rel. 8/9 employs a combination of open-loop and closed-loop control. A mode of operation involves setting a coarse operating point for the transmission power density spectrum by open-loop means based on path-loss estimation. Faster operation can then be applied around the open-loop operating point by closed-loop power control. This controls interference and fine-tunes the power settings to suit the channel conditions including fast fading.
With this combination of mechanisms, the power control scheme in LTE Rel. 8/9 provides support for more than one mode of operation. It can be seen as a toolkit for different power control strategies depending on the deployment scenario, the system load and operator preference.
The detailed power control formulae are specified in LTE Rel. 8/9 for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRS) in section 5.1 in 3GPP TS 36.213, “Physical layer procedures”, version 8.8.0, available at http://www.3gpp.org and incorporated herein by reference. The formula for each of these uplink signals follows the same basic principles; in all cases they can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signaled by the eNodeB, and a dynamic offset updated from sub-frame to sub-frame.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all user equipments in the cell (measured in dBm) and a UE-specific offset, and an open-loop path-loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the MCS and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications as ΔTF, where TF stands for “Transport Format”) allows the transmitted power per resource block to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes: accumulative TPC commands (available for PUSCH, PUCCH and SRS) and absolute TPC commands (available for PUSCH only). For the PUSCH, the switch between these two modes is configured semi-statically for each UE by RRC signaling—i.e. the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level. Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment.
In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells.
The setting of the UE Transmit power PPUSCH [dBm] for the PUSCH transmission in reference sub-frame i is defined by (see section 5.1.1.1 of 3GPP TS 36.213):PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO—PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}  Equation 1                PCMAX is the maximum UE transmit power chosen by the UE in the given range (see below);        MPUSCH is the number of allocated physical resource blocks (PRBs). The more PRBs are allocated, the more uplink transmit power is allocated.        P0—PUSCH(j) indicates the base transmission power signaled by RRC. For semi-persistent scheduling (SPS) and dynamic scheduling this is the sum of a cell specific nominal component PO—NOMINAL—PUSCH(j)ε[=126, . . . , 24] and a UE specific component PO—UE—PUSCH(j)ε[−127, . . . , −96]. For RACH message 3: Offset from preamble transmission power        α denotes a cell-specific parameter (that is broadcast on system information). This parameter indicates how much path-loss PL is compensated. α=1 means the received signal level at eNodeB is same regardless of the user equipment's position, i.e. near cell edge or at centre. If the path-loss is fully compensated, degradation to the cell-edge data rate is avoided. For SPS and dynamic scheduling αε{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}, and for the case of RACH Message 3, α(j)=1.        PL is the UE path-loss derived at the user equipments based on Reference Signal Received Power (RSRP) measurement and signaled Reference Signal (RS) transmission power. PL can be defined as PL=reference signal power−higher layer filtered RSRP.        ΔTF is a modulation and coding scheme (transport format) dependent power offset. It thus allows the transmitted power per resource block to be adapted according to the transmitted information data rate.        f(i) is a function of the closed loop power control commands signaled from the eNodeB to the UE. f( ) represents accumulation in case of accumulative TPC commands. Whether closed loop commands are accumulative (each TPC command signals a power step relative to the previous level) or absolute (each TCP command is independent of the sequence of previous TPC commands) is configured by higher layers. For the accumulative TPC commands two sets of power step values are provided: (−1,1) dB for DCI format 3A and (−1,0+1,+3) dB for DCI format 3. The set of values which can be signaled by absolute TPC commands is (−4,−1,1,4) dB indicated by DCI format 3.Power Headroom Reporting        
In order to assist the eNodeB to schedule the uplink transmission resources to different user equipments in an appropriate way, it is important that the user equipment can report its available power headroom to eNodeB.
The eNodeB can use the power headroom reports to determine how much more uplink bandwidth per sub-frame a user equipment is capable of using. This helps to avoid allocating uplink transmission resources to user equipments which are unable to use them in order to avoid a waste of resources.
The range of the power headroom report is from +40 to −23 dB (see 3GPP TS 36.133, “Requirements for support of radio resource management”, version 8.7.0, section 9.1.8.4, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The negative part of the range enables the user equipment to signal to the eNodeB the extent to which it has received an UL grant which would require more transmission power than the UE has available. This would enable the eNodeB to reduce the size of a subsequent grant, thus freeing up transmission resources to allocate to other UEs.
A power headroom report can only be sent in sub-frames in which a UE has an UL transmission grant. The report relates to the sub-frame in which it is sent. The headroom report is therefore a prediction rather than a direct measurement; the UE cannot directly measure its actual transmission power headroom for the subframe in which the report is to be transmitted. It therefore relies on reasonably accurate calibration of the UE's power amplifier output.
A number of criteria are defined to trigger a power headroom report. These include:                A significant change in estimated path loss since the last power headroom report        More than a configured time has elapsed since the previous power headroom report        More than a configured number of closed-loop TPC commands have been implemented by the UE        
The eNodeB can configure parameters to control each of these triggers depending on the system loading and the requirements of its scheduling algorithm. To be more specific, RRC controls power headroom reporting by configuring the two timers periodicPHR-Timer and prohibitPHR-Timer, and by signalling dl-PathlossChange which sets the change in measured downlink pathloss to trigger a power headroom report.
The power headroom report is send as a MAC Control Element. It consists of a single octet where the two highest bits are reserved and the six lowest bits represent the 64 dB values mentioned above in 1 dB steps. The structure of the MAC Control Element is shown in FIG. 22.
The UE power headroom PH [dB] valid for sub-frame i is defined by (see section 5.1.1.2 of 3GPP TS 36.213):PH(i)=PCMAX−{10·log10(MPUSCH(i))+P0—PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}  Equation 2
The power headroom is rounded to the closest value in the range [40; −23] dB with steps of 1 dB. PCMAX is the total maximum UE transmit power (or total maximum transmit power of the user equipment) and is a value chosen by user equipment in the given range of PCMAX—L and PCMAX—H based on the following constraints:PCMAX—L≦PCMAX≦PCMAX—H PCMAX—L=min(PEMAX−ΔTC,PPowerClass−MPR−AMPR−ΔTC)PCMAX—H=min(PEMAX,PPowerClass)
PEMAX is the value signaled by the network and ΔTC, MPR and AMPR (also denoted A-MPR—Additional Maximum Power Reduction) are specified in 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception”, version 8.7.0, section 6.2 available at http//www.3gpp.org and incorporated herein by reference.
MPR is a power reduction value, the so-called Maximum Power Reduction, used to control the Adjacent Channel Leakage Power Ratio (ACLR) associated with the various modulation schemes and the transmission bandwidth. An adjacent channel may be for example either another Evolved Universal Terrestrial Radio Access (E-UTRA) channel or an UTRA channel. The maximum allowed power reduction (MPR) is also defined in 3GPP TS 36.101. It is different depending on channel bandwidth and modulation scheme. The user equipment's reduction may be less than this maximum allowed power reduction (MPR) value. 3GPP specifies a MPR test which verifies that the maximum transmit power of a user equipment is greater than or equal to the nominal total maximum transmit power minus the MPR while still complying with the ACLR requirements
The following Table 1 shows the Maximum Power Reduction for UE Power Class 3.
TABLE 1maximum power reduction for UE power class 3Channel bandwidthTransmission bandwidth configuration(resource blocks)1.435101520MPRModulationMHzMHzMHzMHzMHzMHz(dB)QPSK>5>4>8>12>16>18≦116 QAM≦5≦4≦8≦12≦16≦18≦116 QAM>5>4>8>12>16>18≦2
For instance, in case of an allocation for a channel bandwidth of 10 MHz, when allocating more than 12 resource blocks and using QPSK modulation, the MPR applied by the user equipment should be smaller than or equal to 1 dB. The actual MPR applied by the user equipment depends on the implementation of the UE and is thus unknown to the eNB.
As indicated above, AMPR is the Additional Maximum Power Reduction. It is band specific and is applied when configured by the network. As can be seen from the explanations above, PCMAX is UE-implementation specific and hence not known by the eNodeB.
FIG. 23 shows exemplary scenarios for a UE transmission power status and corresponding power headroom. On the left hand side of FIG. 23, the user equipment is not power limited (positive PHR), whereas on the right hand side of FIG. 23 a negative power headroom is implying a power limitation of the user equipment. Please note that the PCMAX—L≦PCMAX≦min(PEMAX,PPowerClass) wherein the lower boundary PCMAX—L is typically mainly dependent on the maximum power reduction MPR and the additional maximum power reduction AMPR, i.e. PCMAX—L≅PPowerClass−MPR−AMPR
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE (Release 8) each sub-frame is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols are thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a sub-frame, assuming that the user allocation can change from sub-frame to sub-frame. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number        
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE Release 8/9 (3GPP LTE) are described in detail in 3GPP TS 36.212, “Multiplexing and channel coding (Release 9)”, version 8.8.0 or 9.0.0, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference).
Downlink & Uplink Data Transmission
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier (“cross-carrier scheduling”). This other, cross-scheduled component carrier could be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        When user equipment is configured to have a Carrier Indication Field (CIF) in the L1/L2 control signaling this information identifies the component carrier for which the specific control signaling information is intended. This enables assignments to be sent on one component carrier which are intended for another component carrier. This other, cross-scheduled component carrier may be for example a PDCCH-less component carrier, i.e. the cross-scheduled component carrier does not carry any L1/L2 control signaling.        L1/L2 control signaling for uplink grants is sent on the DL component carrier that is linked with the uplink component carrier or on one of the several DL component carriers, if several DL component carriers link to the same UL component carrier.        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different flavors how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e. is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Physical Downlink Control Channel (PDCCH)
The physical downlink control channel (PDCCH) carries the L1/L2 control signaling, i.e. transmit power control commands and the scheduling grants for allocating resources for downlink or uplink data transmission. To be more precise, the downlink control channel information (i.e. the DCI contents, respectively, the L1/L2 control signaling information) is mapped to its corresponding physical channel, the PDCCH. This “mapping” includes the determination of a CRC attachment for the downlink control channel information, which is a CRC calculated on the downlink control channel information being masked with an RNTI, as will explained below in more detail. The downlink control channel information and its CRC attachment are then transmitted on the PDCCH (see 3GPP TS 36.212, sections 4.2 and 5.3.3).
Each scheduling grant is defined based on Control Channel Elements (CCEs). Each CCE corresponds to a set of Resource Elements (REs). In 3GPP LTE, one CCE consists of 9 Resource Element Groups (REGs), where one REG consists of four REs.
The PDCCH is transmitted on the first one to three OFDM symbols within a sub-frame. For a downlink grant on the physical downlink shared channel (PDSCH), the PDCCH assigns a PDSCH resource for (user) data within the same sub-frame. The PDCCH control channel region within a sub-frame consists of a set of CCE where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE (Release 8/9), a PDCCH can aggregate 1, 2, 4 or 8 CCEs. The number of CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size, etc. Multiple PDCCHs can be transmitted in a sub-frame.
Downlink control channel information in form of DCI transports downlink or uplink scheduling information, requests for aperiodic Cal reports, or uplink power control commands for one RNTI (Radio Network Terminal Identifier). The RNTI is a unique identifier commonly used in 3GPP systems like 3GPP LTE (Release 8/9) for destining data or information to a specific user equipment. The RNTI is implicitly included in the PDCCH by masking a CRC calculated on the DCI with the RNTI—the result of this operation is the CRC attachment mentioned above. On the user equipment side, if decoding of the payload size of data is successful, the user equipment detects the DCI to be destined to the user equipment by checking whether the CRC on the decoded payload data using the “unmasked” CRC (i.e. after removing the masking using the RNTI) is successful. The masking of the CRC code is for example performed by scrambling the CRC with the RNTI.
In 3GPP LTE (Release 8) the following different DCI formats are defined:                Uplink DCI formats:                    Format 0 used for transmission of UL SCH assignments            Format 3 is used for transmission of TPC commands for PUCCH and PUSCH with 2 bit power adjustments (multiple UEs are addressed)            Format 3A is used for transmission of TPC commands for PUCCH and PUCCH with single bit power adjustments (multiple UEs are addressed)                        Downlink DCI formats:                    Format 1 used for transmission of DL SCH assignments for SIMO operation            Format 1A used for compact transmission of DL SCH assignments for SIMO operation            Format 1B used to support closed loop single rank transmission with possibly contiguous resource allocation            Format 1C is for downlink transmission of paging, RACH response and dynamic BCCH scheduling            Format 1D is used for compact scheduling of one PDSCH codeword with precoding and power offset information            Format 2 is used for transmission of DL-SCH assignments for closed-loop MIMO operation            Format 2A is used for transmission of DL-SCH assignments for open-loop MIMO operation                        
For further information on the LTE physical channel structure in downlink and the PDSCH and PDCCH format, see Stefania Sesia et al., “LTE—The UMTS Long Term Evolution”, Wiley & Sons Ltd., ISBN 978-0-47069716-0, April 2009, sections 6 and 9.
Blind Decoding of PDCCHs at the User Equipment
In 3GPP LTE (Release 8/9), the user equipment attempts to detect the DCI within the PDCCH using so-called “blind decoding”. This means that there is no associated control signaling that would indicate the CCE aggregation size or modulation and coding scheme for the PDCCHs signaled in the downlink, but the user equipment tests for all possible combinations of CCE aggregation sizes and modulation and coding schemes, and confirms that successful decoding of a PDCCH based on the RNTI. To further limit complexity a common and dedicated search space in the control signaling region of the LTE component carrier is defined in which the user equipment searches for PDCCHs.
In 3GPP LTE (Release 8/9) the PDCCH payload size is detected in one blind decoding attempt. The user equipment attempts to decode two different payload sizes for any configured transmission mode, as highlighted in Table 1 below. Table 1 shows that payload size X of DCI formats 0, 1A, 3, and 3A is identical irrespective of the transmission mode configuration. The payload size of the other DCI format depends on the transmission mode.
TABLE 1DCI Formatspayload sizetransmissionpayload size Xdifferent from Xmode0/1A/3/3A1Cbroadcast/unicast/paging/power control1Mode 1DL TX modes1Mode 22AMode 32Mode 41BMode 51DMode 61Mode 71Mode 1SPS-Modes1Mode 22AMode 32Mode 41Mode 7
Accordingly, the user equipment can check in a first blind decoding attempt the payload size of the DCI. Furthermore, the user equipment is further configured to only search for a given subset of the DCI formats in order to avoid too high processing demands.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved in the 3GPP. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
In Carrier Aggregation (CA), two or more Component Carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100 MHz. All component carriers can be configured to be 3GPP LTE Release 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. This does not necessarily mean that all component carriers need to be compatible to 3GPP LTE (Release 8/9).
A user equipment may simultaneously receive or transmit on one or multiple component carriers. On how many component carriers simultaneous reception/transmission is possible, is depending on the capabilities of a user equipment.
A 3GPP LTE (Release 8/9) compatible user equipment can receive and transmit on a single CC only, provided that the structure of the CC follows the 3GPP LTE (Release 8/9) specifications, while a 3GPP LTE-A (Release 10) compatible user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a UE with more uplink component carriers that downlink component carriers. In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected mode. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells). The characteristics of the downlink and uplink PCell are:                The uplink PCell is used for transmission of Layer 1 uplink control information        The downlink PCell cannot be de-activated        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        The downlink PCell cell can change with handover        Non-access stratum information is taken from the downlink PCelll.        
The reconfiguration, addition and removal of component carriers can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure component carriers for usage in the target cell. When adding a new component carrier, dedicated RRC signalling is used for sending the system information of the component carrier, the information being necessary for component carrier transmission/reception (similarly as in Rel-8/9 for handover).
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always activate. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling.
The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier. FIGS. 7 and 8 exemplarily show possible linkages between downlink and uplink component carriers. While in FIG. 7 all downlink component carriers are linked to the same uplink component carrier, in FIG. 8 downlink component carriers 1 and 2 are linked to uplink component carrier 1 and downlink component carrier 3 is linked to uplink component carrier 2.
DRX and Carrier Aggregation
In order to provide reasonable battery consumption of user equipment 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX).
For this concept the following terms describe the user equipment's state in terms of DRX.                on-duration: duration in downlink sub-frames that the user equipment waits for, after waking up from DRX, to receive PDCCHs. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer;        inactivity-timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH, failing which it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions).        active-time: total duration that the user equipment is awake. This includes the “on-duration” of the DRX cycle, the time user equipment is performing continuous reception while the inactivity timer has not expired and the time user equipment is performing continuous reception while waiting for a downlink retransmission after one HARQ RTT (Round Trip Time). Based on the above the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite);        
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern, i.e. the same DRX operation applies to all configured and activated component carriers (e.g. identical active time for PDCCH monitoring). When in active time, any component carrier may always schedule PDSCH on any other configured and activated component carrier (further restrictions FFS).
In order to allow for further battery saving optimization when carrier aggregation is configured, a further step of activation/deactivation of component carriers for SCells is introduced. Essentially a downlink component carrier could be in one of the following three states: non-configured, configured but deactivated and active. When a downlink SCell is not active (i.e. configured but deactivated or not configured), the user equipment does not need to receive the corresponding PDCCH or PDSCH, nor is it required to perform CQI measurements. Conversely, when a downlink SCell is active, the user equipment shall receive PDSCH and PDCCH (if present), and is expected to be able to perform CQI measurements. After configuration of component carriers in order to have PDCCH and PDSCH reception on a downlink component as described above, the downlink component carrier needs to be transitioned from configured but deactivated to active state.
Other details of the activation/deactivation mechanism for secondary component carriers may be:                Explicit activation of downlink SCells is done by MAC signaling        Explicit deactivation of downlink SCells is done by MAC signaling        Implicit activation of downlink SCells is also possible        Downlink SCells can be activated and deactivated individually, and a single activation (deactivation command can activate/deactivate a subset of the configured downlink secondary SCells        SCells added to the set of configured component carriers are initially “deactivated”Uplink Power Control for Carrier Aggregation        
Even though most details of the uplink power control algorithm for the carrier aggregation case are still open or under discussion in the 3GPP working groups, the general agreement is that LTE-A Rel. 10 supports component carrier specific uplink power control, i.e. there will be one independent power control loop for each uplink component carrier configured for the user equipment. Furthermore it was decided that power headroom should be reported per-component carrier. In case of power limitation, i.e. UE transmission power is exceeding the total maximum UE transmit power, the following power scaling is applied.
For power scaling, the PUCCH power should be prioritized and the remaining power may be used by PUSCH (i.e. PUSCH power is scaled down first, maybe to zero). Further, a PUSCH with uplink control information (UCI) is prioritized over PUSCH without UCI. Additionally, equal power scaling for PUSCH transmissions without UCI is considered.
As each component carrier can be assumed to have its own power control loop and each transport block on each component carrier is transmitted with a power individually set for the component carrier, power headroom reporting should be performed per component carrier. Since carrier aggregation can be seen as a multiplication of several LTE Rel. 8/9 (component) carriers, it can be assumed that also the power headroom reporting on the individual component carriers will reuse the LTE Rel. 8/9 power headroom reporting procedures.
Furthermore, in LTE Rel.10 within the scope of carrier aggregation there are two maximum power limits, a total maximum UE transmit power PCNMAX and a component carrier-specific maximum transmit power PCMAC,c. 3GPP RAN4 working group already indicated that both (nominal) maximum transmit power per user equipment PCNMAX and the (nominal) maximum component carrier-specific transmit power PCMAC,c should be the same regardless of the number of carriers supported, in order not to affect the link budget of a carrier aggregation capable user equipment in the single carrier operation mode. RAN1 agreed that a power headroom report, which is reported per component carriers, accounts for the maximum power reduction (MPR); in other words, the power reduction applied by the user equipment is taken into account in the component carrier specific maximum transmission power Pcmax,c, where c denotes the component carrier.
Different to LTE Rel. 8/9, in LTE-A Rel. 10 the user equipment has also to cope with simultaneous PUSCH-PUCCH transmission, multi-cluster scheduling, and simultaneous transmission on multiple component carriers, which requires larger MPR values and also causes a larger variation of the applied MPR values compared to 3GPP Rel. 8/9.
It should be noted that the eNodeB does not have knowledge of the power reduction applied by the user equipment on each component carrier, since the actual power reduction depends on the type of allocation, the standardized MPR value and also on the user equipment implementation. Therefore eNodeB doesn't know the component carrier-specific maximum transmission power relative to which the user equipment calculates the power headroom. In LTE Rel. 8/9 for example the user equipment maximum transmit power PCNMAX can be within some certain range as described above (PCMAX—L≦PCMAX≦PCMAX—H).
Due to the reduction of the component carrier-specific maximum transmission power PCMAC,c, which is not known to eNodeB as explained above, the eNodeB cannot really know how close a user equipment is operating to its total maximum transmission power PCNMAX. Therefore there might be situations where user equipment is exceeding the total user equipment maximum transmission power PCNMAX which would hence require power scaling. FIG. 24 shows an exemplary scenario where user equipment is power limited, i.e. applying power scaling on component carriers CC#1 and CC#2 configured in the uplink. Even though the user equipment is power limited, the component carrier-specific power headroom reports according to the LTE definitions indicate sufficiently large power headroom.
In general power scaling might be rather an exceptional case. Therefore, it may be advantageous to design the power headroom reporting procedure such that those power limit cases are reduced. For example as mentioned in the application European patent application 09013756.3) a UE specific power headroom report can be introduced in addition to the component carrier specific power headroom reports, e.g. UE specific power headroom report is reported together with the component carrier specific power headroom reports in order to provide eNB the full picture on UEs power status.
For user equipment power-saving purposes, it's crucial that additional component carriers can be de-activated and activated in an efficient and fast way. With bursty data-transmission, it is imperative that additional component carriers can be activated and de-activated quickly, such that both the gains of high bit-rates can be utilized, and battery preservation can be supported. As described before user equipments will not perform and report CQI measurements on configured but deactivated downlink component carriers but only radio resource management related measurements like RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality) measurements. Hence when activating a downlink component carrier, it's important that eNodeB acquires quickly CQI information for the newly activated component carrier(s) in order to being able to select an appropriate MCS for efficient downlink scheduling. Without CQI information eNodeB doesn't have knowledge about user equipment's downlink channel state and might only select a rather conservative MCS for downlink data transmission which would in turn lead to some resource utilization inefficiency.
In order to acquire CQI information quickly, eNodeB can schedule an aperiodic CQI by means of an uplink scheduling grant. The aperiodic CQI would be transmitted on the physical uplink shared channel (PUSCH). Therefore in order to activate a configured downlink component carrier, eNodeB would need to issue essentially two grants (PDCCH) to the UE, one downlink PDCCH in order to convey the activation command of a downlink component carrier and one uplink PDCCH which schedules uplink resources for the transmission of the aperiodic CQI. Furthermore both PDCCH has to be sent respectively received in the same TTI in order to ensure, that user equipment measures and reports CQI information for the correct downlink component carrier, i.e. the downlink component carrier which will be activated.
The correct reception of the aperiodic CQI can serve as an acknowledgement for the downlink activation command, i.e. when aperiodic CQI has been received eNodeB assumes that user equipment has activated the downlink component carrier indicated in the downlink PDCCH.
As it becomes apparent, the main drawback of the above described component carrier activation method is, that two PDCCHs are required in order to activate a downlink component carrier. Furthermore due to the fact that the two PDCCHs need to be received/sent simultaneously, certain error cases may occur in the presence of PDCCH loss.
In case only the downlink “activation” PDCCH is lost, user equipment will not activate the downlink component carrier. However based on received CQI information eNB erroneously assumes downlink activation has succeeded. In the second error case when only the uplink PDCCH which requests the aperiodic CQI is lost, eNodeB doesn't acquire CQI and erroneously assumes that downlink activation has failed.
There are also problems associated with the power headroom reporting according to 3GPP LTE Rel. 8/9 specification. As already set out before, a power headroom report according to 3GPP LTE Rel. 8/9 specification can only be sent in subframes in which a UE has an uplink assignment for transmission on the PUSCH (transport block), since the power headroom indicates the difference between the nominal user equipment's maximum transmit power and the estimated power for the assigned uplink transmission on the PUSCH. When using component carrier aggregation, no power headroom reporting can be performed for a component carrier which is not scheduled, i.e. without uplink resource assignment in the respective subframe. This is due to the fact that if there is no PUCCH/PUSCH transmission, the component carrier specific maximum transmission power cannot be determined. There is obviously also no transmission format without uplink transmission, i.e. allocated resource blocks, MCS.
However, the eNodeB uses the power headroom reports for determining how much more uplink bandwidth per subframe a UE is capable of using. When the eNB decides to schedule an uplink transmission on said non-scheduled uplink component carrier, the eNB would need to roughly estimate the power situation of said uplink component carrier or blindly schedule future uplink transmissions on the component carrier, since there is no power headroom information for the unscheduled uplink component carrier. Naturally, this blind scheduling may be far from optimum and may thus lead to a waste of resources in the uplink.